In LTE (Long Term Evolution) and successor mobile communication systems of LTE (referred to as, for example, “LTE-Advanced,” “FRA (Future Radio Access),” “4G,” etc.), a user terminal (UE: User Equipment) transmits and receives user data (user (“U”)-plane) and control signals (control (“C”)-plane) to and from a core network (“CN,” also referred to as “EPC” (Evolved Packet Core) and so on) via a radio base station (eNB: eNodeB) (see, for example, non-patent literature 1).
As shown in FIG. 1, in the user plane, a user terminal, a radio base station and a device on the core network (for example, a serving gateway (S-GW), a packet data network gateway (P-GW (Packet data network GateWay)/PDN-GW), etc.) transmit and receive user data. Also, in the control-plane, a user terminal, a radio base station and a device on the core network (for example, the above-noted S-GW, P-GW, a mobility management entity (MME), etc.) transmit and receive control signals.